Due to increasing environmental concerns there exists a need for coatings with low solvent emissions. A growing number of states are placing restrictions on the relative amounts of emitted solvents permitted for various types of coating applications. A VOC (volatile organic content) of less than 2.0 pounds per gallon of product, as applied, is desirable to meet the most demanding restrictions for solvent-containing coatings. It has proven very difficult to apply a solvent-containing coating at VOC levels of 2.0 pounds per gallon or less and still achieve acceptable appearance in a decorative coating. This is especially true of polychromatic coatings, such as automotive body finishes.
Water-based coatings can satisfy most VOC requirements. However, water-based coatings are unsuitable for certain purposes. In particular, water-based coatings cannot be used to supply a stable coating containing a water-reactive metal, such as aluminum.
Water-reducible, solvent-containing coatings have been proposed having a binder system comprising an acrylic polymer having amine-neutralized carboxyl groups plus an amine-formaldehyde resin cross-linking agent. Such compositions are described, for example, in U.S. Pat. No. 4,530,958, the teachings of which are incorporated herein by reference. Although such compositions have worked well for many coating applications, high viscosities of such compositions have made it difficult to apply these compositions at high solids levels. Polymers achieving water solubility through amine neutralization of their carboxyl groups yield relatively high viscosity in water compared to their viscosity in organic solvents.
There exists a continued need for water-reducible coatings which provide low VOC's and which may be applied at high solids levels.